Normally when copying Keychain items from one to the other, the Keychain Access program will ask you for your password for each and every item. Here's a time saving workaround.

First, have the destination keychain (which may be the login keychain) and the source keychain, which you want to merge the items from, both opened up in /Applications/Utilities/Keychain Access.

Now change the password of the destination keychain to be empty. You can do this by selecting the keychain, going to the Edit menu, and choosing the item to change the password. You will need the current password (probably your login password) to change this.

The Keychain Access program will probably complain that the new empty password is not secure enough; but you can override this by holding the Option key and clicking OK.

Next, select all items you want to copy to your destination keychain and drag them over. Since there is no password set, you can just click allow for each item. Huge time saver.

[crarko adds: I tested this, and it works as described. Since it is the destination keychain that you removed the password from, remember to reset it (back to whatever you had to type in to edit the password) when you are through.]

You can also do this using Mobile Me.
Sure, you have to pay for Mobile Me, but a lot of people do, so it's handy to know that you can easily merge keychains using the service.

If you have your old keychain on one Mac and your new keychain on another, on each Mac set your Mobile Me sync preferences to sync keychains.
It will ask you for the other keychain password once on each Mac and then your two keychains will be merged.

Crarko, 2 additional notes.
First, do not set the password for the destination to blank. Open (doubleclick) the keychain you want to import and change that password to blank. Now you can easily drag all its items to your own (login) keychain. The password dialog is from the keychain you want to acces i.e. want to copy from.
Second. i thought you need the option-key to overrule the warning for empty/unsecure password. As it turns out, the warning shows up once, if you click ok again it will set the password, even without the option key.

My original hint has been edited by someone. The information is now wrong!
Don't set the password of the destination keychain to empty, that is of no use.

1. first, make a copy of the keychain you want to copy items from,
2. Double click or open this copy keychain in keychain-access
3. set the password for this copy keychain to empty
4. drag the items from the copy keychain over, to your own keychain
5. remove and delete the copy keychain